1. Field of the Invention
The present invention relates to a directional coupler using dielectric lines as transmission paths, an antenna device incorporating the directional coupler, and a radar system including the antenna device.
2. Description of the Related Art
A directional coupler using dielectric lines as transmission paths is disclosed in Japanese Unexamined Patent Application Publications Nos. 8-8621 and 10-200331.
Japanese Unexamined Patent Application Publication No. 8-8621 is related to a directional coupler which uses a non-radiative dielectric waveguide (hereinafter referred to as xe2x80x9cNRD guidexe2x80x9d). Because of its low transmission loss in a single NRD guide, the LSM mode is used as a transmission mode in a coupling portion of the directional coupler. A bent portion has a radius of curvature of one of several discrete values so as to provide lower loss. The directional coupler is adapted to propagate electromagnetic waves in both the LSM mode and the LSE mode. Therefore, problems arise in that mode switching is likely to occur in the directional coupling portion, resulting in ripples in the insertion loss versus frequency characteristic.
Japanese Unexamined Patent Application Publication No. 10-200331 is directed to an antenna device incorporating a directional coupler which uses dielectric lines as transmission paths, in which the secondary line is moved parallel to the primary line to achieve beam scanning. A gap between the two lines of the directional coupler forms a choke, thereby preventing leaky wave loss. However, when the directional coupler is adapted to propagate electromagnetic waves in the LSM mode and the LSE mode, loss resulting from mode switching occurs, as in the directional coupler disclosed in Japanese Unexamined Patent Application Publication No. 8-8621. If the electromagnetic waves are propagated solely in the LSM01 mode as a primary mode, there are also problems in that the electromagnetic waves are likely to leak from the gap between the primary line and the secondary line, possibly increasing the insertion loss.
Accordingly, the present invention provides a compact directional coupler which solves the problems of increased insertion loss due to mode switching in the coupling portion of the primary line and the secondary line which form the directional coupler, which has improved design flexibility in the bent portion, and which suppresses leakage of the electromagnetic waves from the gap between the primary line and the secondary line of the directional coupler when they are separated from each other.
The present invention further provides a compact antenna device incorporating a compact directional coupler having lower loss, which achieves high rate beam scanning, and provides a compact radar system having a high detection ability using the antenna device.
To this end, a directional coupler includes two non-radiative dielectric lines, each having flat conductive surfaces placed substantially in parallel to each other, and a dielectric strip disposed therebetween, the two non-radiative dielectric lines being coupled to each other so that at least portions of the dielectric strips are close to and extend in parallel to each other. The main transmission mode of electromagnetic waves at the frequency used is an LSE mode, the electromagnetic waves being propagated in the non-radiative dielectric lines. The LSE mode is used as a main transmission mode, thereby maintaining low loss and realizing a compact directional coupler.
Preferably, the cross-sectional dimension of the dielectric strips and the spacing between the flat conductive surfaces are defined so that electromagnetic waves at the frequency used may be propagated solely in the LSE mode in the non-radiative dielectric lines. Therefore, the loss caused by mode switching between the LSE mode and the LSM mode in the bent portion can be suppressed.
The two non-radiative dielectric lines which form the directional coupler may be separated by separating surfaces extending along the longitudinal direction of the two dielectric strips, and the two non-radiative dielectric lines may be placed in the longitudinal direction of the dielectric strips so as to be relatively displaced with respect to each other. Therefore, the two non-radiative dielectric lines can be relatively displaced with respect to each other while they are coupled to each other, thereby reducing the loss due to leakage of electromagnetic waves from the separating surfaces.
Each of the two non-radiative dielectric lines may include conductive plates which hold the dielectric strip, and the opposing surfaces of the conductive plates, which correspond to the separating surfaces of the non-radiative dielectric lines, preferably have choke grooves formed therein. This reliably suppresses leakage of the electromagnetic waves in the LSE mode from a gap between the opposing surfaces of the conductive plates.
In another aspect of the present invention, an antenna device includes a primary emitter connected to one of two non-radiative dielectric lines in a directional coupler which are separated from each other, and a dielectric lens which substantially focuses onto the primary emitter. Therefore, the primary emitter can be relatively displaced with respect to the dielectric lens when the two non-radiative dielectric lines in the directional coupling portion are relatively displaced, thereby achieving high rate beam scanning.
In still another aspect of the present invention, a radar system includes a unit for transmitting and receiving electromagnetic waves, and the unit includes the above-described antenna device. Therefore, the overall radar system becomes compact since it incorporates an antenna device including a compact and light-weight directional coupler, and can achieve high rate beam scanning.
Other features and advantages of the present invention will become apparent from the following description of embodiments of the invention which refers to the accompanying drawings.